Magnetic and ferroelectric properties of oxygen octahedron/tetrahedron mixed ultrathin multiferroic layer by oxygen desorption

BiFeO3 epitaxial films (tBFO = 1–10 nm) were grown on a LaSrMnO3 (tLSMO = 70 nm) buffer layer and capped with a Ru or Ru/Al layer. The BiFeO3 ultrathin film (tBFO = 3 nm) was confirmed to be continuous and flat over a wide area. Piezoelectric response and switching of spontaneous polarization were observed in the ultrathin films by switching-spectroscopy piezoelectric force microscopy measurements. Using the lock-in method with a modulation frequency of 10 Hz in x-ray absorption spectroscopy (XAS), a difference was observed between the left- and right-circularly polarized XAS signals of Fe L3 and L2 owing to x-ray magnetic circular dichroism (XMCD). The number of Fe L3 and L2 XMCD peaks indicated that a mixture of oxygen octahedron and tetrahedron (divalent and trivalent) structures might be formed. The tBFO dependence of XMCD revealed that approximately 1 nm thick ferro-like magnetic properties emerged on the BiFeO3 side of the BiFeO3/Ru interface. Ru may partially extract oxygen from BiFeO3, forming an oxygen tetrahedron structure. The BiFeO3/Ru bilayer exhibits both a piezoelectric response and ferro-like magnetism, which is promising as the basic structure of multiferroic tunnel junctions.